(a) Technical Field
The present invention relates to a positive chargeable magnetic toner composition providing the extended life of the drum, reduction of the background contamination (fogging image), and improvement of long-term reliability.
(b) Description of the Related Art
In general, the dry-process developing systems in electrophotography can be largely classified into dual-component developing system using a dual-component developer comprising a toner and a carrier, and mono-component developing system using a mono-component developer comprising a toner only. The mono-component developing system is advantageous in compactness, low cost, and easy maintenance. Recently, the copier and printer adopting mono-component developing system are widely spread, and the printing speed is notablely improved.
Differing from the dual-component toner comprising carrier particles transferring toner particles, the fluidity of toner particles greatly affects the transfer characteristics of toner in the non-magnetic mono-component toner.
In the non-magnetic mono-component developing system, the thickness of toner layer on the developing roller is controlled by pressing the developing roller with metal or resin blade. In the dual-component developing system, the toner is transferred to the developing roller by charging the toner with friction between the toner and carrier.
However, in case of the magnetic mono-component developing system, the toner is transferred to developing roller by using magnetic force as driving force. That is, doctor blade is arranged so as to make contact with a developing roller, and the mono-component toner is triboelectrically charged by passing between doctor blade and developing roller. The charge toner is maintained on the surface of the developing roller by electrostatic force.
A charged toner is used for visualizing the latent image on drum. If organic photo conductor (OPC) drum which is manufactured by coating at least an organic layer is repeatedly contacted with toner on its surface in a long time, it is difficult to form an image because of the abrasion of OPC surface. Such problem increases fogging image in non-imagining region and makes image density (i.e., blackness) insufficient.
In the prior art, to prevent the lower image quality caused by drum surface abrasion, Japanese Patent Laid-Open No. H10-326028 discloses a method of using alumina particle in combination of silica with high hardness which is used for obtaining the fluidity of toner. In addition, Japanese Patent Laid-Open No. H11-153886 discloses a positive chargeable color toner containing a urethane-modified polyester resin as binder resin in toner particle.
However, the prior arts do not effectively reduce the abrasion of drum surface, because the large amount of silica can not be reduced or used. In addition, if the amount of added silica is excessively reduced, the image density is insufficient because the decreased fluidity and increased coadhesion force of toner decreases the transfer efficiency. There is practical difficulty in preventing the abrasion of drum surface, and obtaining the high image density at the same time.
Therefore, it is still required to provide a positive chargeable toner which extends the drum life by reducing the abrasion of developing drum surface despite of copying in a long period of time, reduces the background contamination (fogging image by obtaining the excellent triboelectrification, and improves the long-term reliability by maintaining the high image density.